The present invention relates to a torque distribution control system for a four-wheel drive motor vehicle having a central differential, and more particularly to the torque distribution control system of a vehicle with a laterally mounted engine.
In a four-wheel drive motor vehicle, the torque distribution ratio between front wheels and rear wheels of the vehicle is determined to coincide with a dynamic weight ratio of the front and rear wheels, so that power of the engine is effectively used at acceleration.
In the four-wheel drive motor vehicle based on a front-wheel drive with a front-mounted engine which has the static weight ratio of 60 (front):40 (rear), the ratio of the front torque T.sub.F to the front wheels and rear torque T.sub.R to the rear wheels is determined in the ratio of 50:50 which is the same as the dynamic weight ratio thereof. In the four-wheel drive motor vehicle based on a rear-wheel drive with a front-mounted engine which has the static weight ratio of 50:50, the ratio of the front torque T.sub.F and the rear torque is determined 40:60 in the same ratio as the dynamic weight ratio. Accordingly, a central differential comprising bevel gears is employed in the former vehicle in which an output torque of the engine is equally distributed, and the central differential having a simple planetary gear device is employed in the latter vehicle.
The former vehicle ensures safe driving on a slippery road. If a differential lock device is provided for locking the central differential, the driving force of the vehicle is improved further. However, steering ability of the vehicle is not particularly improved. That is, when the vehicle makes a turn at high speed under the differential lock condition, all the four wheels may slip (slipping spin) at the same time, causing difficulty in driving.
In order to ensure driving stability of the vehicle, the torque to the rear wheels is set to a larger value than that to the front wheels by arranging the central differential comprising the simple planetary gear device, so that the rear wheels may slip first. Thus, the vehicle can be safely driven by the front wheels at a small torque while the rear wheels idle.
Japanese Patent Application Laid-Open 63-176728 discloses a four-wheel drive motor vehicle in which a central differential comprising a simple planetary gear device is provided. An output of a transmission is transmitted to a carrier of the planetary gear device. The torque is distributed to the front wheels through either a sun gear or a ring gear and to the rear wheels through the other gear. The torque to the front and rear wheels is unequally distributed at the ratio determined by the difference between the pitch circles of the sun gear and the ring gear. A fluid-operated multiple-disk friction clutch as a lock device is provided for controlling the differential operation. A standard torque distribution ratio determined by the ratio of the pitch circles can not be changed, unless the diameters of the sun gear and the ring gear change.
In order to increase the standard torque distribution ratio, the diameter of the sun gear is decreased or that of the ring gear is increased. However, in a power transmitting system where an axle differential for front or rear axles and the central differential are coaxially disposed, a plurality of shafts such as axles, a front drive shaft, an input shaft connected to the transmission and a rear drive shaft are disposed so as to penetrate the sun gear. Therefore, the diameter of the sun gear cannot be decreased. On the other hand, the ring gear cannot be large because of a limitation of space and of the required gear ratio. Consequently, the power transmitting system cannot be applied to various vehicles having different static weight ratios, such as a front-wheel drive vehicle with a front-mounted engine, a rear-wheel drive vehicle with a rear-mounted and a mid-shipped engine.
Moreover, since the distribution of the torque to the rear wheels can not be set to a large value, the control range of the multiple-disk friction clutch becomes small. Consequently, a motor vehicle having good driveability and steering ability can not be provided.
In a four-wheel drive vehicle having a laterally mounted engine, the central differential and the fluid-operated multiple disk clutch are usually coaxially disposed with the front axle differential, as is in the prior art, thereby rendering the system compact. In addition, the same parts can be used for both transaxles of a two-drive wheel vehicle and a four-wheel drive vehicle. However, such an arrangement inevitably cannot be freely modified into variant designs. Moreover, since a large torque at the final gear is directly transmitted to the central differential, the central differential must be sufficiently rigid and durable. In addition, in a power transmission system where the engine is laterally mounted, a transfer device for changing the direction of the power train must be provided, so that the system must be arranged taking into the account of the space and the strength thereof.